fix(share): robust clipboard copy with an iOS-safe fallback

navigator.clipboard.writeText is intermittent on iOS Safari (bug 9: the
download share-link copy 'often does neither'). Add lib/clipboard.ts
copyText(): a gesture-safe execCommand path FIRST (before any await can
spend the transient user activation), async Clipboard API as fallback.
Migrate all five copy call sites to it.

frontend/src/components/ShareDialog.tsx

@@ -6,6 +6,7 @@ import clsx from "clsx";
 import Modal from "./Modal";
 import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";
+import { copyText } from "../lib/clipboard";
 import { api, type DownloadJob, type ShareLink } from "../lib/api";
 import { inputCls, btnCls } from "./ui/form";
 const EXPIRY_DAYS = [null, 1, 7, 30] as const;
@@ -87,11 +88,10 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
   const fullUrl = window.location.origin + link.url;
 
   const copy = async () => {
-    try {
-      await navigator.clipboard.writeText(fullUrl);
+    if (await copyText(fullUrl)) {
       setCopied(true);
       setTimeout(() => setCopied(false), 1500);
-    } catch {
+    } else {
       notify({ level: "error", message: t("downloads.share.copyFailed") });
     }
   };